1. Field of the Invention
The present invention relates to an ink jet head for use in an ink jet printer and a method of manufacturing the ink jet head.
2. Description of Prior Art
Japanese Patent Publication No. 61-59914 discloses one such conventional thermal jet type ink jet printer. With this type of prior art printer, bubbles are developed in the ink by a heater disposed within the ink pressure chambers and the bubbles exert a pressure on the ink causing ink drops to be ejected through the orifices.
However, the ink tends to be deteriorated by the heat generated by the heater so that the printing cannot be effected with the ink in its optimum condition. In addition, the unstable pressure exerted by the bubbles may cause the orifices to be clogged with the ink or the bubbles may enter the ink paths. As a result, printing quality is deteriorated. Further, the structural components of the ink jet head may be cracked due to repetitive thermal stress.
In order to solve the aforementioned drawback, Japanese Patent Preliminary Publications No. 5-338156 and No. 6-8426 propose a piezoelectric shear mode type ink jet head shown in FIGS. 13-19 where the piezoelectric material is formed with grooves therein that serve as ink pressure chambers, the walls defining the grooves are deformed when the walls undergo shear stresses, and the deformation of the walls pressurizes the ink to eject ink drops through the orifices. The ink is ejected by the mechanical deformation of the piezoelectric material and therefore the ink is not deteriorated. The amount of deformation of the piezoelectric material varies with the applied voltage, facilitating control of the pressure in the ink pressure chamber. Therefore, the orifices are not clogged with the ink, or bubbles will not enter the ink paths, improving printing quality.
Controlling the pressures in the ink pressure chambers enables adjusting of the diameters of ink drops, lending itself to printing with tone gradations.
The method of manufacturing the prior art piezoelectric shear mode ink jet head will be described.
FIG. 13 illustrates the prior art piezoelectric base before grooves are formed therein. FIG. 14 illustrates a prior art upper base made of a piezoelectric material before grooves are formed therein. FIG. 15 illustrates the prior art ink jet head when it is being assembled. FIG. 16 is a side view of the prior art ink jet head. FIG. 17 is a fragmentary front view of the prior art ink jet head before an orifice plate is assembled to the ink jet head.
Referring to the figures, a block 12A of a piezoelectric material is polarized in a direction shown by arrow P and is formed with an electrode 11 that extends over the entirety of its surface. A block 14A of a piezoelectric material is polarized in a direction shown by arrow P and is formed with an electrode 13 that extends over the entirety of its surface. The blocks 12A and 14A are formed with first grooves 16 and second grooves 17, respectively, in their surfaces, using a cutting tool such as a dicing saw, not shown. The first and second grooves 16 and 17 are formed at predetermined intervals and having the same width. The block 12A having the first grooves 16 is used as a piezoelectric base 12 and the block 14A having the second grooves 17 is used as a piezoelectric base 14.
The piezoelectric bases 12 and 14 are connected together with an electrically conductive adhesive member 19 therebetween, the grooves 16 and 17 defining ink pressure chambers 24a, 24b, . . . (only chambers 24a and 24b are shown). An orifice plate 25 is bonded to one ends of the piezoelectric bases 12 and 14, and a sealing member 26 is provided on the other ends. Thus, the ink pressure chambers 24a and 24b are sealed against the environment except through the orifices 28a and 28b formed in the orifice plate 25 and a common ink chamber 29 formed at a rear end of the piezoelectric base 14.
Referring to FIG. 17, the grooves 16 divide the electrode 11 into electrodes 31a, 31b, . . . (only electrodes 31a and 31b are shown) and the grooves 17 divide the electrode 13 into electrodes 20a, 20b, . . . (only electrodes 20a and 20b are shown). Walls 32a, 32b, . . . (only walls 32a and 32b are shown) in the piezoelectric base 12 cooperate with walls 33a, 33b, . . . (only walls 33a and 33b are shown) in the piezoelectric base 14 to define the ink pressure chambers 24a and 24b when the bases 12 and 14 are placed together.
When a positive voltage +V and a negative voltage -V are applied to the electrode 31a and 31b, respectively, by a drive circuit, not shown, electric fields are developed in the piezoelectric bases 12 and 14 in directions shown by arrows E, resulting in shear mode deformation in the walls 32a and 33a and walls 32b and 33b. The directions of shear mode deformation in the walls are opposite as depicted by dotted lines in FIG. 17 so that the ink, not shown, in the ink pressure chamber 24a is pressurized. As a result, the ink drops are ejected through the orifice 28a.
Ink used in printers is usually water-soluble and water-soluble ink has a smaller electric resistance than oily ink. A leakage current that flows through the ink between the electrodes 31a and 31b not only prevents a desired shear mode deformation from occurring but can cause damages to the drive circuit. In order to solve this problem, a coating layer 35 in the form of an electrically insulating film is formed on the inner walls of the ink pressure chambers 24a and 24b. The method of applying the coating layer 35 includes dipping, spin-coating, and chemical vapor deposition (CVD).
In the dipping method, the piezoelectric bases 12 and 14 are connected together and then the orifice plate 25 is bonded to the piezoelectric bases 12 and 14, thereby defining ink chambers 24a and 24b. Thereafter, the ink jet head is immersed in an insulating liquid, not shown. The insulating liquid enters the ink pressure chambers 24a and 24b due to capillary phenomenon through the orifices 28a, 28b, . . . (only orifices 28a and 28b are shown) and is deposited to the inner walls of the ink pressure chambers 24a and 24b. The insulating liquid deposited on the inner walls of the ink pressure chambers 24a and 24b is allowed to cure, thereby forming the coating layer 35.
In the spin-coating method, the insulating liquid is first introduced into the ink pressure chambers 24a and 24b by the dipping method or other method, and is then spin-removed by a centrifugal force. Then, the thin film of the insulating liquid is allowed to cure, thereby forming the coating layer 35 on the walls of the ink pressure chamber.
In the CVD method, the ink jet head is placed in a furnace and an insulating material is evaporated, thereby forming insulating layers on the walls of the ink pressure chambers.
However, with the prior art ink jet heads, the diameters of the orifices 28a and 28b are very small, e.g., approximately 30 microns, and it is difficult for the insulating liquid to go through the orifices into the chambers. Therefore, the insulating liquid cannot be applied completely over the inner walls of the ink pressure chambers 24a and 24b. In addition, the insulating liquid can cure in the orifices, leaving the orifices closed.
In order to solve this problem, the following two manufacturing methods may be used.
In the first prior art manufacturing method, the coating layer 35 is formed after the piezoelectric bases 12 and 14 are laminated together, and then the orifice plate 25 is bonded.
FIG. 18 illustrates the first manufacturing method of the prior art ink jet head, the orifice plate being omitted for explanation and FIG. 19 is a cross-sectional view taken along lines 19--19 in FIG. 18.
Referring to FIGS. 18 and 19, the piezoelectric bases 12 and 14 are laminated together and then the coating layer 35 is formed. Relative positional errors between the piezoelectric bases 12 and 14 results when the piezoelectric bases 12 and 14 are laminated together. Therefore, the piezoelectric bases 12 and 14 are subjected to lapping operation using lapping paper, not shown, so that one ends of the piezoelectric bases becomes flush with each other. Subsequently, the orifice plate 25 is bonded to the ends of the piezoelectric bases 12 and 14.
However, when the lapping operation is performed for the ends of the piezoelectric bases 12 and 14, forces are exerted in a direction normal to the plane in which the coating layer 35 is formed, causing the coating layer 35 to come off the wall of the ink pressure chamber near the orifice plate 25. Therefore, when the ink pressure chambers are filled with ink 37, the ink 37 directly come into contact with the electrodes 31a and 31b making it difficult to electrically insulate the electrodes 31a and 31b from each other.
In the second prior art manufacturing method, the lapping operation is performed to make the ends of the piezoelectric bases 12 and 14 flush with each other, then coating operation is performed to apply an insulating film on the walls of the ink pressure chambers, and finally the orifice plate 25 is bonded to the piezoelectric bases 12 and 14.
FIG. 20 illustrates the second manufacturing method of the prior art ink jet head.
The piezoelectric bases 12 and 14 are connected together and then the lapping operation is performed to make the ends of the bases 12 and 14 flush with each other using lapping paper, not shown. Then, the coating operation is performed to form the coating layer 40 on the piezoelectric bases 12 and 14. The coating layer 40 is applied not only to the ends of piezoelectric bases 12 and 14 but to the inner surface (depicted at 35 in FIG. 19) of the ink chambers 24a and 24b.
Then, the orifice plate 25 is bonded to the ends of the piezoelectric bases 12 and 14.
Insulating materials are usually chemically very stable and therefore highly electrically insulating, and shows high resistances against corrosion by the ink 37. On the other hand, the coating layer 40 tends to repel water so that the bonding agent 41 applied to the coating layer 40 loses its bonding effect to a certain degree when the piezoelectric bases 12 and 14 are bonded together by the bonding agent 41.
As a result, the ink pressure chambers 24a and 24b become less airtight, resulting in poor ejection of ink drops.